The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is para-xylene, the principal feedstock for polyester which continues to enjoy a high growth rate from large base demand. Ortho-xylene is used to produce phthalic anhydride, which has high-volume but mature markets. Meta-xylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene is selectively disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered.
A current objective of many petrochemical and aromatics complexes is to increase the yield of xylenes and to de-emphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to convert toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,016,219 discloses a process for toluene disproportionation using a catalyst comprising a zeolite which has been modified by the addition of phosphorus in an amount of at least 0.5 mass-%. The crystals of the zeolite are contacted with a phosphorus compound to effect reaction of the zeolite and phosphorus compound. The modified zeolite then may be incorporated into indicated matrix materials. U.S. Pat. No. 4,097,543 discloses toluene disproportionation for the selective production of para-xylene using a zeolite which has undergone controlled pre-coking. The zeolite may be ion-exchanged with a variety of elements from Group IB to VIII, and composited with a variety of clays and other porous matrix materials.
U.S. Pat. No. 4,182,923 discloses a process for toluene disproportionation with a high conversion of the toluene to benzene and para-xylene by use of an aluminosilicate zeolite of silica to alumina ratio above 12 and which has been modified by treatment with ammonium hydrogen phosphate to deposit phosphorus. U.S. Pat. No. 4,629,717 discloses a phosphorus-modified alumina hydrogel formed by gelation of a homogeneous hydrosol. The composite has a relatively high surface area of 140 to 450 m2/g and high activity and selectivity in 1-heptene conversion tests.
U.S. Pat. No. 6,114,592 discloses an improved process combination for the selective disproportionation of toluene. The combination comprises selective hydrogenation of a toluene feedstock followed by a zeolitic catalyst. U.S. Pat. No. 6,359,185 discloses an oil-dropped zeolitic catalyst in an amorphous aluminum phosphate binder that enhances selectivity.
U.S. Pat. No. 6,191,331 discloses a pre-coking method that avoids a large temperature rise by using a low pressure in the presence of nitrogen and a low ratio of hydrogen-to-hydrocarbon. U.S. Pat. No. 6,429,347 discloses that running a process at a hydrogen-to-hydrocarbon ratio between 0.2 and 0.5 improves the selectivity of para-xylene and decreases the selectivity of benzene.
Workers in the field of aromatics disproportionation continue to seek processes and catalysts having exceptionally high conversion to para-xylene from toluene combined with favorable selectivity and stability.